It's pretty much universally accepted that healthy humans have sentience, but that's where the agreement stops. Many would say that complex animals have sentience, such as dogs, cats, cows, sheep, etc. But what about a mouse, an ant, or a bacterium? And what about a severely brain-damaged human that doesn't seem to be self-aware, and yet is conscious? Or, what about a developing embryo—does it go from being non-sentient to sentient at some point?

This is a significant issue for moral philosophers to address, and one that has plagued them for ages. It is also a hotly debated issue in the realm of animal rights, where sentience is commonly cited as the justification for exempting an animal from undergoing unnecessary suffering. Consider, for example, this passage from the English philosopher Jeremy Bentham:

But a full-grown horse or dog is beyond comparison a more rational, as well as a more conversable animal, than an infant of a day, or a week, or even a month, old. But suppose the case were otherwise, what would it avail? The question is not Can they reason? nor, Can they talk? but, Can they suffer?

So, my question is at what point does it make sense to say that a creature has 'gained sentience', and how do we measure it? And furthermore, once a being has gained sentience, can they lose it if they become severely brain damaged?

A couple of possible criteria come to mind: number of brain cells, the ability to experience emotion, self-awareness (the ability to recognize oneself). Are any of these independently sufficient? Or are there multiple criteria required to declare sentience? Are there other important concerns?

@Matthew I think that is basically what his question is.
– ChadJun 13 '11 at 19:46

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Upvoted - I think that although this question is general and invites argumentative answers, it is well phrased and should provoke interesting responses
– ChuckJun 14 '11 at 13:11

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@smartcaveman: I've incorporated your clearly philosophical link into the question, and re-opened it. Just as a note, only one person can be notified using @comments. It's always the first person addressed in your comment. So addressing all of the people who voted to close doesn't really work. You make a reasonable argument here; that's not how I originally read the question.
– Cody GrayJul 3 '11 at 13:32

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I agree with Joseph. I do not think that, as written, this question is a good example of the sort of material intended on this site. While there are aspects of the content that are certainly redeemable, I think that this presentation is in want of focus. As stated (philosophy.stackexchange.com/faq), questions should not be chatty or open-ended, if at all possible.
– davidlowrydudaJul 4 '11 at 4:35

1 Answer
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This is a controversial topic, with various conflicting view points. In general, sentience at least requires consciousness and the ability to feel pleasure and pain.

The Stanford Encyclopedia of Philosophy article on animal consciousness gives a good, non-biased account of what criteria for sentience is compatible with which epistemological and ontological positions. The issue is broken down into two particular questions, which are discussed thoroughly in the article:

Can we know which animals beside humans are conscious? (The Distribution Question)

Can we know what, if anything, the experiences of animals are like? (The Phenomenological Question)

If you are interested in learning about theories of sentience when divorced from their ethical implications, you will benefit from reading about Sellars' distinction between "sentience" and "sapience". This is summarized in section 7 of the Stanford Encyclopedia of Philosophy article on Consciousness and Intentionality.